This invention relates to fixation devices for the spinal column. More particularly, this invention relates to a bone screw and associated components that may also be referred to as a pedicle screw assembly.
The spine is formed of a series of bones called vertebrae. There are 33 vertebrae, which are grouped as cervical, thoracic, lumbar, sacral, and coccygeal vertebrae, according to the regions of the spine they occupy. A typical vertebra consists of two essential parts, an anterior segment or body, and a posterior part, or vertebral or neural arch. These two parts enclose a foramen, the vertebral foramen. Together, the vertebral foramen of the vertebrae form a canal for the protection of the spinal cord. The vertebral arch consists of a pair of pedicles and a pair of laminae.
The body is the largest part of a vertebra, and is more or less cylindrical in shape. Its upper and lower surfaces are flattened. In front, the body is convex from side to side and concave from above downward. Behind, it is flat from above downward and slightly concave from side to side. The pedicles are two short, thick processes, which project backward, one on either side, from the upper part of the body, at the junction of its posterior and lateral surfaces.
Over the years, various techniques and systems have been developed for correcting spinal injuries and/or degenerative spinal processes. Spinal correction frequently requires stabilizing a portion of the spine to facilitate fusing portions of the spine or other correction methodologies. Medical correction of this type is frequently employed for many spinal conditions, such as, for example, degenerative disc disease, scoliosis, spinal stenosis, or the like. Frequently, these corrections also require the use of implants, such as, bone grafts. Stabilizing the spine allows bone growth between vertebral bodies such that a portion of the spine is fused into a solitary unit.
Several techniques and systems have been developed for correcting and stabilizing the spine and facilitating fusion at various levels of the spine. In one type of system, a rod is disposed longitudinally along the length of the spine in the region of concern. The rod is arranged according to the anatomy and the correction desired. In this system, the rod is aligned along the spine and engages various vertebrae along its length. The rod engages, or more typically, a pair of parallel rods engage the spine using fixation elements, such as anchors, attached to vertebral bodies by a bone screw that is inserted into the pedicle and penetrates into the body of the vertebra.
Anatomy and correction frequently require aligning the rod and screw at various angles along the length of the portion of correction. In order to provide this alignment, polyaxial screws/anchors have been developed. Many variations of bone screw and rod fixation systems exist on the market today. However, prior systems have been limited in the amount of angulation permitted relative to the place of attachment to the spine. Additionally, prior systems have involved the securing of a screw assembly to a support rod by direct contact between a set screw and the rod. This contact causes subtle damage to the support rod caused by plastic deformation of the rod by the set screw.
Therefore, there is a need for a bone screw assembly that permits a wide range of angulation relative to the place of attachment to the spine and the support rod while providing an effective and secure lock of the screw and rod in the desired position.
There is also a need for a mechanism of attachment of a spinal stabilizer or support rod to a bone screw that minimizes the possibility of damage to the rod by a set screw securing the rod to the bone screw.
There is also the need for a bone screw adapted to allow for placement into the pedicle and subsequent attachment of a housing to the bone screw in minimally invasive surgery.